1. Field of the Invention
The present invention relates to a flat cathode-ray tube which is employed for an image display unit such as a television or a display, and more particularly, it relates to a highly reliable vacuum sealing structure which is attained by reducing stress in a joint portion between a front glass vessel and a back metal vessel.
2. Background of the Invention
FIG. 21 schematically shows an exemplary structure of a conventional flat cathode-ray tube. Referring to FIG. 21, numeral 1 denotes a cathode portion serving as an electron beam source, numeral 2 denotes an electron beam control part which is formed by at least one or more electrode plates (2A and 2B in FIG. 21) for controlling an electron beam generated from the cathode portion 1, and numeral 3 denotes a front glass vessel provided with a fluorescent screen 4 which is hit with the electron beam. A vacuum housing 6 is formed by the front glass vessel 3 and a back metal vessel 5. At least one (2A in FIG. 21) of the electrode plates forming the electron beam control part 2 is fixed to a frame 7 and suspended by pins 8 which are mounted on the front glass vessel 3 through springs 9, while the remaining electrode part (2B in FIG. 21) is fixed by joint fittings 13 which are mounted on a side surface portion of the back metal vessel 5. Numeral 10 denotes wiring terminals which are drawn out from the vacuum housing 6 from a joint surface between the front glass vessel 3 and the back metal vessel 5. Numeral 11 denotes a ceramic coating which is plasma-sprayed to the back metal vessel 5, and numeral 12 denotes low melting point glass (frit glass) joining the front glass vessel 3 and the back metal vessel 5 with each other. Numeral 13 denotes the joint fittings for mounting the cathode portion 1 on the back metal vessel 5, numeral 14 denotes electric signal input terminals for the cathode portion 1 etc., and numeral 15 denotes a metal exhaust pipe. Numeral 16 denotes internal wires for electrically connecting the electric signal input terminals 14 with the cathode portion 1, and numeral 18 denotes a wiring substrate forming a wiring pattern by a metal conductive film.
The operation principle of the flat cathode-ray tube shown in FIG. 21 is now briefly described. A prescribed voltage is applied from the electric signal input terminals 14 to the cathode portion 1 in the structure shown in FIG. 21, so that an electron beam is generated from the cathode portion 1. Further, a potential is applied from an external power supply circuit (not shown) to the electron beam control part 2 through the wiring substrate 18 for accelerating or modulating the electron beam which is generated from the cathode portion 1, so that the electron beam correctly hits a determined position of the fluorescent screen 4 provided on the front glass vessel 3. This operation is repeated to visually reproduce images.
In the conventional flat cathode-ray tube shown in FIG. 21, the respective side surface portions of the front glass vessel 3 and the back metal vessel 5 are strongly joined with each other through the frit glass 12. In employment, a general cathode-ray tube enters an ultrahigh vacuum state of not more than 10.sup.-5 Pa, with occurrence of extremely high stress.
Further, the vessel is deformed due to the vacuum pressure, as a matter of course. When the bottom surface is flat as shown in FIG. 21, the back metal vessel 5 is extremely deformed to cause remarkable stress in the joint portion between the same and the front glass vessel 3. Thus, sufficient connection strength cannot be attained and the structure is insufficient for serving as a vacuum housing.
In the conventional structure, in addition, stress on the front glass vessel 3 is also increased and the cathode-ray tube is unsatisfactory as a vacuum housing. Particularly in the structure receiving the wiring terminals 10, the joint portion between the front glass vessel 3 and the back metal vessel 5 is disadvantageous in reliability.
In a general flat cathode-ray tube, further, it is important to minimize the overall depth thereby reducing its thickness.